1. Field of the Invention
This invention relates in general to circuits and more specifically to transistor body bias control circuits.
2. Description of the Related Art
Transistors are used to implement circuitry in an integrated circuit. For some transistors, such as some types of Field Effect Transistors, carriers (e.g. holes or electrons) move in a channel region between a source and a drain of the transistor when a voltage above a threshold voltage is applied to the gate of the transistor. Typically, the channel region is located in a doped semiconductor well.
With some transistors, both the switching speed of the transistor and the sub threshold leakage current are determined by the threshold voltage. For FETs, a higher threshold voltage corresponds to a slower switching speed and lower leakage current. A lower threshold voltage corresponds to a faster switching speed but with higher leakage current. The amount of leakage current affects the power consumption of the transistor where a higher leakage current corresponds to higher power consumption.
The threshold voltage of a transistor (and correspondingly the leakage current, power, and switching speed) can be adjusted by biasing the body of the transistor (typically the well were the channel region is located) at a voltage different than the voltage applied to the source of the transistor (source voltage). Reverse body biasing can be used to raise the threshold voltage of a transistor. With an NFET, reverse body biasing is performed by applying a voltage to the body that is less than the voltage applied to the source (typically system ground (VSS) in some examples). For a PFET, reverse body biasing is performed by applying a voltage to the body that is greater than the voltage applied to the source (typically VDD in some examples).
Forward body biasing can be used to lower the threshold voltage of a transistor. For an NFET, forward body biasing is performed by applying a voltage to the body that is higher than the voltage applied to the source. For a PFET, forward body biasing is performed by applying a voltage to the body that is less than the voltage applied to the source.
In some examples, a rise in ambient temperature causes more leakage current during operation. Hence a transistor operating in a hotter environment may generate a greater amount of leakage current than one operating in a cooler environment.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The Figures are not necessarily drawn to scale.